Other factors in weight gain

In addition to air pollution, gastrointestinal bacteria and sleep deprivation, a number of other overlooked factors that may affect weight have been identified by researchers:

Maternal age, weight

Women are waiting longer to have children, and children of older women are more likely to become obese. Overweight pregnant women, meanwhile, can unwittingly pre-program their offspring for obesity, research shows.

More carbon dioxide in the atmosphere

According to one hypothesis, outlined in the journal Nutrition & Diabetes, inhaling increased amounts of carbon dioxide makes the blood more acidic (or decreases its pH level). In the brain, a lower pH level may cause appetite-related neurons to fire more frequently, leading to increased appetite and decreased activity.

Indoor heat

Colder temperatures can activate brown adipose tissue, which burns rather than stores calories. According to a study published last year in the journal Obesity Reviews, room temperatures have increased in both the U.S. and Britain since the 1960s. In mildly cold conditions (such as a chilly room), people generate extra heat without shivering. This may trigger brown fat to consume calories to keep the body warm. Studies suggest that increased time spent in warm conditions may lead to a loss of brown fat and a reduced capacity to burn energy.

Air conditioning

When it's hot, people eat less. Air conditioning could also keep people indoors, reducing outdoor exercise.

Endocrine disrupters

Chemical compounds that mimic or block the body's hormonal actions, including bisphenol A and phthalates, have been linked to obesity in young children. Research suggests prenatal exposure to some chemicals may affect weight later in life.

Viruses

Adenoviruses cause respiratory or gastrointestinal infections. One called adenovirus 36 has been linked to weight gain; a study found it may infect fat cells, making them grow more quickly and multiply in greater numbers than normal. Further research is needed.

Pharmaceutical drugs

Weight gain is associated with several commonly used medicines, including drugs for depression, diabetes, allergies, hypertension, contraception and mental illness.

Decreased smoking

In the short term, nicotine increases energy expenditure and may reduce appetite. This could explain why light smokers tend to have lower body weight than do nonsmokers and why people often gain weight after quitting.